Polymers and numerous additives are typically compounded into formulations and subsequently cross-linked for enhanced strength properties of the finished article. Additives which can be added into the formulation, depending upon the application, include oil, fillers (e.g., carbon black, talc, magnesium hydroxide, or calcium carbonate), co-agents (e.g., triallyl cyanurate) and cross-linking agents (usually peroxide). These formulations are first compounded and mixed thoroughly to enable the various ingredients to compatibilize, since the compositions usually contain a relatively high percentage of filler. In the case of a wire and cable coating operation, the compositions coat the wire and are subsequently cross-linked to stabilize the composition.
The formulated composition must have physical properties which are often mutually exclusive, depending upon the choice of the polymer. For example, the composition must have "green strength" to remain on the wire after coating, and not sag or deform on the wire until the composition is cured, otherwise the wire will have thin spots and the insulating value of the composition is lost. The composition must also undergo a cure step and maintain good physical properties, such as tensile strength, elongation, and 100% modulus (stress at 100% strain).
Usually the polymer selected to compatibilize all of the various components is an elastomer such as ethylene/propylene rubber (EPM) or ethylene/propylene diene monomer terpolymer (EPDM). These types of very low density polymers are relatively expensive (as compared with traditional linear low density polyethylene polymers) and contain a very high percentage by weight of co-monomer(s). Lowering the density of the polymer increases the ability of the polymer to hold more filler and oil. But as the polymer density decreases, the modulus of the polymer decreases, the overall composition loses uncured modulus (100% modulus) or "green strength" and performs unsatisfactorily.
There have been a few recent announcements regarding new polymers which are said to be effective substitutes for EPM and EPDM. Union Carbide Chemicals and Plastics Inc. announced in 1990 that they have developed a new cost effective class of polyolefins trademarked Flexomer.TM. Polyolefins that could replace expensive EPM or EPDM rubbers. These new polyolefins are said to have bridged the gap between rubbers and polyethylene, having moduli between the two ranges.
In a paper presented on Sep. 22-27, 1991 at the 1991 IEEE Power Engineering Society Transmission and Distribution Conference ("New Specialty Linear Polymers (SLP) For Power Cables," printed in the proceedings on pp. 185-190) in Dallas, Tex., Monica Hendewerk and Lawrence Spenadel, of Exxon Chemical Company, report that Exxon's Exact.TM. polyolefin polymers, said to be produced using single site catalyst technology, are useful in wire and cable coating applications. The new polymers are linear and said to have narrow molecular weight distributions, and, because of the narrow molecular weight distribution, are also said to have "the potential for melt fracture." The physical properties of filled and cross-linked formulations comprising Exact.TM. polymers are favorably compared with formulations comprising EP polymers in this publication. In a similar vein, in "A New Family of Linear Ethylene Polymers Provides Enhanced Sealing Performance" by Dirk G. F. Van der Sanden and Richard W. Halle, (February 1992 Tappi Journal), Exxon Chemical Company has also taught that the molecular weight distribution of a polymer is described by the polymers melt index ratio (i.e., I.sub.10 /I.sub.2 ) and that their new narrow molecular weight distribution polymers made using a single site catalyst are "linear backbone resins containing no functional or long chain branches."
While the development of new lower modulus polymers such as Flexomer.TM. Polyolefins by Union Carbide or Exact.TM. polymers by Exxon has aided the elastomeric formulation marketplace, there continues to be a need for other more advanced, cost-effective polymers for compounding with reasonable loadings of various fillers which improves or maintains physical properties such as "green strength" and also improves cross-linked properties such as tensile strength.